osMessageGet(main_msgBox, osWaitForever) with respect to CPU sleep

nRF52832, SD 132, RTOS, SDK13

Can anyone let me know how CPU will do when waiting for a message like below?

evt = osMessageGet(main_msgBox, osWaitForever); //wait for q box forever

If CPU is running actively and waiting for a message, how can I put the CPU in low power mode while I still can receive a message from other threads?

Thanks.